The present invention relates to an electrochemical cell employing a solid polymer electrolyte membrane (ion exchange membrane) such as a fuel cell, an electrolytic cell, a sensor and the like in which supply of a reaction gas and discharge of an obtained gas can be smoothly conducted to obtain an elevated limiting output in the practical range and to a process of preparing same.
Since a solid polymer electrolyte electrochemical cell, for example, a solid polymer electrolyte membrane fuel cell is much compact and can take out a higher current density than a phosphoric acid fuel cell, it is attracting much attention as an electric source of an automobile and a space craft. Also in the development in this technical field, various proposals for electrode structures, processes of preparing a catalyst and system constitutions have been made. FIG. 1 schematically shows a principle and a constitution of a polymer electrolyte fuel cell in which an anode side gas diffusion electrode 4A consisting of an anode side porous catalyst layer 2A and an anode side hydrophobic porous current collector layer 3A bonded with each other is bonded to one surface of an ion exchange membrane 1, and a cathode side gas diffusion electrode 4C consisting of a cathode side porous catalyst layer 2C and a cathode side hydrophobic porous current collector layer 3C bonded with each other is bonded to the other surface of the ion exchange membrane 1. A separator 6A having reaction gas supply grooves 5A is in contact with the anode side gas diffusion electrode 4A and current collecting portions 7A are constituted between the adjacent supply grooves 5A of the separator 6A. Similarly, a separator 6C having reaction gas supply grooves 5C is in contact with the cathode side gas diffusion electrode 4C and current collecting portions 7C are constituted between the adjacent supply grooves 5C of the separator 6C. The fuel cell is prepared by hotpressing the five components. By connecting the both current collector portions 7A and 7C with a lead having a load 8 and supplying hydrogen to the anode and oxygen to the cathode, electric power can be taken out through the load 8.
FIG. 2 is an enlarged sectional view of the fuel cell of FIG. 1 showing the exemplified fine structure of the anode and cathode catalyst layers 2A, 2C.
In this conventional polymer electrolyte fuel cell, electroconductive unwoven fabric hydrophobically treated is employed as the above current collector not for preventing the gas flow as much as possible. However, especially in a high current density operation in which the mass transfer is rate-determining, the gas supply is insufficient even with the unwoven fabric so that since the reverse direction mass transfers, that is, the supply of an oxidation gas and the discharge of water vapor must be performed on a single plane, the supply of the oxidation gas is likely to be insufficient resulting in the lowering of the voltage in the high current density region.
Further as the support of the catalyst layer of the fuel cell, carbon particles are employed which are mixed with ion exchange resin and integrated by means of hotpressing or the like to form the catalyst layer. The catalyst layer thus formed is packed making no spaces, and because the carbon particles are spherical and are firmly adhered with one another by means of the hotpressing to increase its density, the number of gas flow paths is small so that a reaction gas is difficult to diffuse in the catalyst layer and to reach to a reaction point and in addition the discharge of water vapor after the reaction cannot be smoothly conducted. Accordingly, the supply of the reaction gas becomes further difficult to largely lower the reaction efficiency on the catalyst layer to only take out energy significantly lower than the maximum output.